Flavylium-Supported Poly(<i>N</i>‑isopropylacrylamide): A Class of Multistimuli Responsive Polymer

A new multistimuli responsive polymer was prepared by means of the free-radical copolymerization of <i>N</i>-isopropylacrylamide and a vinyl flavylium derivative. The polymer shows response to temperature (conferred by the <i>N</i>-isopropylacrylamide units) and pH and light stimuli (conferred by the flavylium moieties). The polymer follows the typical pH dependent network of chemical reactions of flavylium compounds in water that was studied by means of UV–vis spectroscopy and stationary fluorescence emission. Regarding the temperature response, the presence of the flavylium cation in the polymeric chains reduces the LCST, and it is possible to take advantage of this effect to photo-collapse partially the polymer by irradiating the system in defined ranges of pH and temperature

Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium

2-Phenyl-1-benzopyrylium (flavylium) and 2-styryl-1-benzopyrylium (styrylflavylium) cations establish in aqueous solution a series of equilibria defining chemical reaction networks responsive to several stimuli (pH, light, redox potential). Control over the mole fraction distribution of species by applying the appropiate stimuli defines a horizontal approach to supramolecular chemistry, in agreement with the customary bottom-up approach toward complex systems. In this work, we designed an asymmetric styrylchalcone able to cyclize in two different ways, producing two isomeric styrylflavylium cations whose chemical reaction networks are thus interconnected. The chemical reaction networks of 2-(2,4-dihydroxystyryl)-1-benzopyrylium (AH⁺) and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium (AH⁺_iso) comprise the usual species observed in flavylium-derived networks, in this case, the styryl derivatives of quinoidal bases, hemiketals, and chalcones. The thermodynamics and kinetics of the crossed networks were characterized by the use of UV–vis absorption and NMR spectroscopy as well as time-resolved pH jumps followed by stopped-flow. The two styrylflavylium cations are connected (isomerize) through two alternative intermediates, the asymmetric <i>trans</i>-styrylchalcone (Ct) and a spiropyran-type intermediate (SP). At pH = 1, AH⁺ slowly evolves (<i>k</i>_obs ≈ 10^–5 s^–1) to a mixture containing 62% AH⁺_iso through the Ct intermediate, while at pH = 5, the SP intermediate is involved. The observed rate constants for the conversion of the styrylflavylim cations into equilibrium mixtures containing essentially Ct follow a pH-dependent bell-shaped curve in both networks. While at pH = 1 in the dark, AH⁺ evolves to an equilibrium mixture containing predominantly AH⁺_iso, irradiation at λ > 435 nm induces the opposite conversion

Design and Synthesis of Photoactive Ionic Liquids

Two ionic liquids with photoisomerizable <i>p</i>-hydroxycinnamic acid moieties were synthesized and characterized by X-ray crystallography and DSC, and their photochemistry was studied in solution and neat conditions. Irradiation at absorption maxima led to <i>trans–cis</i> photoisomerization and resulted in significant reduction of melting temperatures of the ionic liquids. X-ray structures of both compounds show an intricate network of supramolecular interactions before irradiation. Physical and chemical transformations are completely reversible upon irradiation at lower wavelengths of ionic liquid solutions in acetonitrile

Photorheological Ionic Liquids

Two room temperature ionic liquids (ILs) bearing coumarin and <i>p-</i>hydroxycinnamic acid moieties are synthesized, and their photochemistry is studied in solution and neat conditions. Irradiation at absorption maxima leads to photochemical transformations and results in changes of their rheological properties which are evaluated by rotational rheometry. Samples of ionic liquids are also studied by Hyper-Rayleigh scattering, and the effect of their photochemistry on ionic nanoaggregation is discussed

Acenaphthylene-Based Chromophores for Dye-Sensitized Solar Cells: Synthesis, Spectroscopic Properties, and Theoretical Calculations

A set of acenaphthylene dyes with arylethynyl π-bridges was tested for dye-sensitized solar cells (DSSCs). Crucial steps for the extension of the conjugated system from the acenaphylene core involved Sonogashira coupling reactions. Phenyl, thiophene, benzotriazole, and thieno-[3,2-b]thiophene moieties were employed to extend the conjugation of the π-bridges. The systems were characterized by cyclic voltammetry and by UV–vis absorption and emission. The spectroscopic characterization showed that the last three bridges resulted in red-shifted absorption and emission spectra relative to the parent phenyl-bridged compound, in accordance with TD-DFT calculations. The phenylethynyl derivative 6a achieved a conversion efficiency of 2.51% with Voc, Jsc, and FF values of 0.365 V, 13.32 mA/cm2, and 0.52, respectively. The efficiency of this compound improved to 3.15% with the addition of CDCA (10 mM), representing the best efficiency result in this study. The overall conversion efficiency of the other aryl derivatives 6b–d proved to be significantly inferior (14–40%) to that of 6a due to a significant decrease of Jsc

Impact of Water on the Cis–Trans Photoisomerization of Hydroxychalcones

The photochromism of a 2-hydroxychalcone has been studied in CH₃CN and H₂O/CH₃OH (1/1, v/v), as well as in analogous deuterated solvents using steady-state (UV–vis absorption, ¹H and ¹³C NMR) and time-resolved (ultrafast transient absorption and nanosecond flow flash photolysis) spectroscopies. Whereas the irradiation of <i>trans</i>-chalcone (<b>Ct</b>) under neutral pH conditions leads to the formation of the same final chromene derivative (<b>B</b>) in both media, two distinct photochemical mechanisms are proposed in agreement with thermodynamic and kinetic properties of the chemical reaction network at the ground state. Following light excitation, the first steps are identical in acetonitrile and aqueous solution: the Franck–Condon excited state rapidly populates the <i>trans</i>-chalcone singlet excited state ¹<b>Ct</b>* (LE), which evolves into a twisted state ¹<b>P</b>*. This excited state is directly responsible for the photochemistry in acetonitrile in the nanosecond time scale (16 ns) leading to the formation of <i>cis</i>-chalcone (<b>Cc</b>) through a simple isomerization process. The resulting <i>cis</i>-chalcone evolves into the chromene <b>B</b> through a tautomerization process in the ground state (τ = 10 ms). Unlike in acetonitrile, in H₂O/CH₃OH (1/1, v/v), the <b>P</b>* state becomes unstable and evolves into a new state attributed to the tautomer ¹<b>Q</b>*. This state directly evolves into <b>B</b> in one photochemical step through a consecutive ultrafast tautomerization process followed by electrocyclization. This last case represents a new hypothesis in the photochromism of 2-hydroxychalcone derivatives

2,2′-Spirobis[chromene] Derivatives Chemistry and Their Relation with the Multistate System of Anthocyanins

The chemistry of 2,2′-spirobis­[chromene] derivatives is intimately related to the one of anthocyanins and similar compounds. The 2,2′-spirobis­[chromene] species plays a central role in the network of chemical reactions connecting two different flavylium-based multistate systems. In the present work, a new asymmetric 2,2′-spirobis­[chromene] intermediate possessing a constrained propylenic bridge between carbons 3 and 3′ was isolated and its role as a pivot in the anthocyanins-type multistate of chemical reactions was investigated by the conjugation of absorption spectroscopy, stopped-flow, NMR, and X-ray crystallography. It was confirmed that the propylenic bridge is essential to stabilize the spirobis­[chromene] species. Furthermore, under acidic conditions, two <i>cis</i>–<i>trans</i> styrylflavylium isomers were identified, which could be interconverted directly into one another with light. This is the first report of styrylflavylium cations with photoisomerization on the styryl moiety